net: ipv6: only invalidate previously tokenized addresses

Instead of invalidating all IPv6 addresses with global scope
when one decides to use IPv6 tokens, we should only invalidate
previous tokens and leave the rest intact until they expire
eventually (or are intact forever). For doing this less greedy
approach, we're adding a bool at the end of inet6_ifaddr structure
instead, for two reasons: i) per-inet6_ifaddr flag space is
already used up, making it wider might not be a good idea,
since ii) also we do not necessarily need to export this
information into user space.

Suggested-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>